Charge output sensors have been in widespread use in recent years. The charge output sensors are electric charge generating sensors, such as strain gauges and acceleration sensors, using piezoelectric elements made of insulating material. Since minor electric charge is detected in such a charge output sensor, it is necessary to provide an amplifier circuit for amplifying a signal detected by the charge output sensor.
In addition, resistors having resistance values of the order of gigaohms are required in semiconductor devices with increase in function and integration in recent years.
In such situations, Patent Document 1 relates to a detection apparatus, a sensor, and an electronic device and discloses an amplifier circuit including an operational amplifier, a resistor, and a capacitor. A non-inverting input terminal of the operational amplifier is grounded. The resistor and the capacitor are electrically connected in parallel between an output terminal and an inverting input terminal of the operational amplifier.
Patent Document 2 relates to a gigaohm-load resistor for a micro-electronic integrated circuit and discloses production of a high-resistance element by using a MOSFET in the weak inversion region.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-224230.
Patent Document 2: International Publication No. 95/25349.
However, a review by the inventor indicates that, in the charge amplifier, the frequency range of the detection signal from the charge output sensor frequently reaches a low-frequency domain and, in such a case, it is necessary to mount a high-resistance element of at least several tens megaohms in order to reduce the cutoff frequency determined by the resistance value of the resistor (feedback resistance) and the capacitance of the capacitor (feedback capacitance).
In the configuration disclosed in Patent Document 1, the mounting of the high-resistance element of several tens megaohms or larger increases the size of the entire circuit configuration. No disclosure and no suggestion are provided about specific configurations to reduce the size of the circuit and to increase the integration.
In the configuration disclosed in Patent Document 2, although the production of the high-resistance element by using a MOSFET in the weak inversion region is disclosed, no disclosure and no suggestion are provided about specific configurations indicating how the high-resistance element is applied to the charge amplifier.
The review by the inventor also indicates that the resistance value of the MOSFET in the weak inversion region of the MOSFET is very sensitive to a variation in a series of manufacturing processes of the MOSFET and a change in power supply voltage and temperature because the resistance value of the MOSFET is exponentially varied depending on the capacity of the oxide film of the MOSFET and factors including threshold voltage and the temperature. In addition, the resistance value of the MOSFET in the weak inversion region is also exponentially varied depending on not only the variation in the gate voltage but also the variation in the drain voltage and the source voltage.
Accordingly, when the MOSFET is activated in the weak inversion region and is applied to the charge amplifier as a pseudo resistor, it is necessary to separately provide an adjustment circuit of the gate voltage in order to adjust the resistance value, that is, a pseudo resistance value of the MOSFET and the pseudo resistance value is also varied when the drain-source voltage of the MOSFET is varied. Consequently, non-linearity as the resistor is strong and waveform distortion may occur in the signal output from the MOSFET with the variation in the power supply voltage and so on.